Hypercalciuria is a risk factor for Ca oxalate/apatite kidney stones. Serum levels of the renal hormone, 1,25-(OH)2-vitamin D, are often elevated in hypercalciuric stone formers reflecting increased synthesis in the kidneys. Elevated serum 1,25-(OH)2-D can result in hypercalciuria by stimulation of intestinal Ca absorption when Ca is available in the diet and/or stimulating bone resorption when dietary Ca intake is low. Ca stone formers often exhibit mild hypophosphatemia and dietary PO4 deprivation is a known stimulus of renal 1,25-(OH)2-D synthesis in animals, possibly acting via a hormone since this response to PO4-deprivation is dependent on normal pituitary function. Ca stone formers also exhibit reduced intestinal Mg absorption. Thus we plan to continue studies of the regulation of renal synthesis and serum 1,25-(OH)2-D levels in healthy volunteers as affected by limiting intestinal PO4 absorption with Al(OH)3 and by dietary Mg-deprivation or loading. Liquid diets reduce serum 1,25-(OH)2-D levels and we will evaluate the role of the source of dietary protein and of fiber on this effect. Hypercalciuric stone formers will be studied to determine the frequency of renal Ca wasting and of increased intestinal Ca absorption independent of 1,25-(OH)2-D. We will also evaluate total and free serum 25-OH-D and 1,25-(OH)2-D levels and serum vitamin D binding protein (VDBG) levels in patients with the nephrotic syndrome and patients undergoing peritoneal dialysis in relation to the losses of VDBG in the vitamin D metabolites into the urine or into peritoneal dialysate. We have established primary cultures of mouse renal cortical cells in serum free media which synthesized 1,25-(OH)2-D as measured by a protein binding assay. We plan to culture the cells on membranes to expose both the basolateral as well as the luminal surfaces to the culture media since recent studies have shown that such a system permits detection of peptide hormone receptors. This in vitro system will be utilized to evaluate renal 25-OH-D3-1Alpha-hydroxylase activity by the Ca and PO4 concentrations and pH of the medium, PTH and calcitonin, intestinal hormones that may be involved in the suppression of 1,25-(OH)2-D synthesis by liquid diets and to search for a possible hormonal stimulus appearing in response to PO4 deprivation or present in the serum of Ca stone formers.